First, pictures would be helpful in this case. Also, what kind of set up do you have -- handlebars, brakes, levers, fork, etc?

There are a few different options. Most of the higher-end bags (Giles Berthod, Rivendell Baggins, Carradice) use some sort of rack underneath to set the bottom of the bag on. The Nashbar looks shorter/smaller than those, so maybe go with tension cords from the front corners to the brake levers to pull the bag out a bit -- a bit like the Rivendell Candybar bag. Surf the retro-grouch websites like wallbike.com or rivbike.com or velo-orange.com and you'll get an idea for how the smaller bags can be secured.

Or maybe if it has a hard plastic back inside then drill some holes and put U-bolts through it to tighten it onto the handlebar (with tube pieces underneath) and keep it from rotating forward?